Article of the month March

Azimuth sundial 3

Vertical cylinder

Mac Oglesby of Vermont, USA, designed a nomogram to calculate the time of day from the azimuth of the sun and the date.
Measurement of the azimuth requires a separate instrument, an example of which was on this site in January 2007 (see the Archive).

Together, the two form an indirect sundial. Here are two examples of such a nomogram.
The first example has a date scale from January up to December; the second has one from July up to June.
This difference in date scale makes the pattern look quite different.

date scale from January to December

date scale from July to June

Meanwhile, Joël Robic of France published on his web site a vertical cylinder sundial that does read time directly from azimuth and date.

This sundial has no gnomon. One of the two shadow lines on the cylinder is the readout line. To this end, noon is turned east or west as required.
The sundial is shown below. Note the elastic band pointing out the current date.

Joël Robic’s idea

It turns out that the patterns by Oglesby and Robic are identical.
In retrospect, that is not surprising. What is surprising is how two persons work on an azimuth dial and arrive at the same pattern simultaneously.

A disadvantage of direct readout on the cylinder is that the shadow line is not sharp.
There are however several solutions to this. An example by Oglesby, using a rotating vertical rod, is shown here.

Do you use the Deltacad CAD package? A Deltacad macro that will draw nomograms is available here:
Download macro